景谷M_S6.6地震同震地表破坏特征与孕震构造

The co-seismic ground failure features and seismogenic structure of the Jinggu M_S6.6 earthquake

无量山断裂带位于云南西南部,主要由磨黑、宁洱、普文和景谷—云仙4条断裂组成,晚第四纪活动特征明显.受青藏高原隆起影响滇西南块体向南运动,中下地壳广泛存在的低速层为块体运动提供了有利条件,但刚性的临沧花岗岩体对其南向运动起着顶托作用,使得东、西两侧块体运动速率出现差异,且块体运动方向与无量山断裂带呈小角度相交.在此背景下,无量山断裂带表现为水平右旋走滑运动,起着滑动分解应变的作用.在其与横向断层交汇部位或在断裂端部,应力易于集中而引发地震,此次MS6.6地震就发生在断裂的端部.据野外科考调查,在宏观震中区集中出现带状砂土液化和地裂缝等地面破坏.喷砂孔呈串珠状线性分布,主要有NW和NE两组;NW向地裂缝呈右阶雁行状、NE向地裂缝呈左阶雁行状排列特征,它们具有明显的构造成因.地震烈度长轴方向、余震分布和震源机制解等显示,此次地震是沿NW向节面右旋走滑所致,宏观地面破坏特征和微观观测结果非常吻合,一致表明此次地震破裂与景谷—云仙断裂运动有关,其孕震构造应是景谷—云仙断裂.

The Wuliangshan fault system in southwestern Yunnan is active during the late Quaternary.This fault system mainly consists of the Mohei,Ning′er,Puwen,and Jinggu-Yunxian faults.The southwestern Yunnan block moves southward as affected by the uplift of the Tibet plateau,and the widespread existence of low velocity zones within the middle and lower crust within this region has provided favorable conditions for facilitating the block movement.To the west of the Wuliangshan,the rigid Lincang granite pluton obstructs the southward motion of the block,leading to the difference in velocity vectors on the eastern and western sides of this pluton block.Such velocity vector difference resulted in an acute angle between the directions of the southwestern Yunnan block and the Wuliangshan fault system.As a result,the Wuliangshan fault system moves right-laterally to partition the regional strain field.At places near the ends of the fault zones or where these faults intersect the transverse faults perpendicular to the Wuliangshan fault system,the stresses tend to be concentrated to induce earthquakes.The M_S6.6Jinggu earthquake just took place at the end of the fault.Based on the results of field investigation for the shock,we have found plenty of ground ruptures,such as clustered linear sand liquefaction features and ground fissures on the first grade terrace in the macro-epicenter area.The sand blasting holes are distributed like strings of beads,trending in NW and in NE directions.The NW-trending fissures are aligned as right-step en echelons,while the NE-trending fissures as left-step en echelons,which requires a tectonic explanation for their formation.The NW-trending long axis of the seismic intensity map,the linear spatial distribution of the aftershocks and the focal mechanism solution,collectively suggest that the M_S6.6Jinggu earthquake is caused by right-lateral slip rupture along the NW nodal plane of the focal mechanism solution.Moreover,the surface rupture features and their agreement with our microscopic survey results reveal that the earthquake rupture features are basically consistent with the NW-trending Jinggu-Yunxian fault movement,suggesting the Jinggu-Yunxian fault as the seismogenic fault.